An organosol containing fibril-forming high-molecular-weight PTFE particles dispersed in an organic solvent is useful as an electrode mixture for producing electrodes of electric storage devices such as lithium cells.
Such An organosol of high-molecular-weight PTFE particles tends to have very low stability when containing the PTFE particles at a high concentration, and therefore the highest concentration thereof is as low as 25% by mass if the stability needs to be maintained. For this reason, attempts have been made to modify PTFE so as to produce a high-concentration organosol.
In this context, Patent Documents 1 to 4 each disclose a method employing core-shell particles each having a core of fibril-forming high-molecular-weight PTFE and a shell of non-fibril-forming polymer. Also, Patent Document 5 teaches a method of modifying PTFE by copolymerization with a very small amount of an acrylic monomer having a polyfluoroalkyl group.
Still, the proportion of the modified PTFE particles in an organosol obtained in those Patent Documents is up to 30% by mass.
Meanwhile, combination use of PTFE particles and other fluororesins has also been proposed. For example, Patent Document 6 teaches a method in which unmodified PTFE and a tetrafluoroethylene (TFE)-hexafluoropropylene (HFP) copolymer (FEP) are used in combination. Patent Document 7 teaches an aqueous dispersion or organosol of a mixture of high-molecular-weight PTFE and FEP or PFA. Patent Document 8 teaches an organosol in which particles of a crystalline fluoropolymer such as high-molecular-weight PTFE and an amorphous fluororesin such as a vinylidene fluoride (VdF) polymer are mixed.
The ratio of PTFE/FEP, however, is at most 30/70 (mass ratio) in Patent Document 6, and a ratio higher than that causes agglomeration. Further, the organosol of Patent Document 7 contains only unmodified PTFE in an amount of 10% by mass or less. Although Patent Document 7 mentions a mixture containing unmodified PTFE and PFA at amass ratio of 50/50, such a mixture is available only in an aqueous dispersion.
The organosol of Patent Document 8 is produced by a latex mixing method of mixing a latex of PTFE particles and a latex of amorphous or low-crystalline fluororesin particles, coagulating and drying the mixture, and dispersing the dried mixture in an organic solvent, or by a dry-blending method of blending dried PTFE particles and dried amorphous fluororesin particles and then dispersing the blended particles in an organic solvent. Here, the dry blending enables production of an organosol containing PTFE in an amount of more than 50% by mass of the solids, but the dry-blended PTFE particles are agglomerated or fibrillated to have large particle sizes and thus do not exist as primary particles, which means that such an organosol lacks stability. On the other hand, in the organosol obtained by latex mixing, the generation rate of secondary particles is still high, which is similar to a conventional one, and the PTFE particle content should be not higher than 20% by mass to allow the organosol to have excellent precipitation stability.
Patent Document 1: JP S62-109846 A
Patent Document 2: JP H2-158651 A
Patent Document 3: JP H4-154842 A
Patent Document 4: WO 96/012764
Patent Document 5: JP S63-284201 A
Patent Document 6: JP S48-27549 B
Patent Document 7: JP H10-53682 A
Patent Document 8: JP 2008-527081 T